There are metal alloys, for example, some aluminum, magnesium and titanium alloys, that display exceptional ductility when deformed under controlled conditions. These aluminum alloys are susceptible to extensive deformation under relatively low shaping forces. Such alloys are characterized as being superplastic. The tensile ductility of superplastic metal alloys typically ranges from 200% to 1000% elongation.
Superplastic alloy sheets are formed by a variety of processes into articles of manufacture that are frequently of complex shape. These superplastic forming (SPF) processes are usually relatively slow, controlled deformation processes that yield complicated products. But an advantage of SPF processes is that they often permit the manufacture of large single parts that cannot be made by other processes such as sheet metal stamping. Sometimes a single SPF part can replace an assembly of several parts made from non-SPF materials and processes.
There is a good background description of practical superplastic metal alloys and SPF processes by C. H. Hamilton and A. K. Ghosh entitled "Superplastic Sheet Forming" in Metals Handbook, Ninth Edition, Vol. 14, pages 852-868. In this text several suitably fine grained, superplastic aluminum and titanium alloys are described. Also described are a number of SPF processes and practices for forming superplastic materials. In these practices gas pressure is often used to stretch or form a sheet, that has been heated to a superplastic forming temperature, into contact with the shaping surface of a suitable tool. For SPF aluminum alloys, this temperature is typically in the range of about 400.degree. C. to 565.degree. C. The rate of gas pressurization is controlled so the strain rates induced in the sheet being deformed are consistent with the required elongation for part forming. Suitable strain rates are usually 0.0001 to 0.01 s.sup.-1.
It is desired to adapt SPF processes to forming relatively large sheets of superplastic aluminum alloys. In order to accomplish this goal, it is necessary to devise methods and equipment to produce complex SPF panels or other parts at large volumes. In connection with this effort it is necessary to devise a sheet blank loader to position, e.g., an SPF aluminum sheet between upper and lower heated tools to pre-bend and heat the sheet preparatory to subjecting it to stretch forming or other suitable superplastic forming process. Such a loading device must function suitably in the SPF processing environment for the sheet blank which in the case of aluminum alloys is typically about 800.degree. F. to 1050.degree. F. (425.degree. C. to 565.degree. C.).
It is an object of this invention to provide a relatively uncomplicated and inexpensive blank loader of such capability.